Abstract
AbstractAgricultural substrates (maize silage and cattle manure) were used to carry out methane fermentation process in bioreactors under laboratory conditions. Identical mixtures of these substrates were incubated for 43 days at 20, 30 and 40ºC to determine how different temperature conditions affect the δ13C(CH4), δH(CH4), and δ13C(CO2) values. To ensure correct anaerobic digestion, the following parameters of the organic substrates and fermentation solutions were monitored: total organic carbon (TOC), volatile solids (VS), volatile fatty acids (VFA), chemical oxygen demand (COD) and carbon to nitrogen ratio (C/N). The variants with higher incubation temperature yielded higher amounts of biogas (20ºC=84.5, 30ºC=101.8 and 40ºC=133.3 dm3/kg VS). In the case of gas products of methane fermentation, it was observed that the higher temperature of incubation affects the depletion in heavy isotopes. At 20ºC, 30ºC, and 40ºC mean values of δ13C(CH4) reached −26.4, −29.7, and −35.4‰, respectively. Mean values of δ2H(CH4) were −311.6, −354.0, and −398.5permil, and of δ13C(CO2) +8.9, +3.7, and −2.3‰, respectively. Moreover, the apparent fractionation coefficient α13C(CO2-CH4) were calculated, which decreased when the temperature increased. This isotopic tool was used to identify acetoclastic reaction as a dominant methanogenesis pathway. Observed changes in the isotopic composition of gaseous products obtained at different incubation temperatures may indicate decomposition of different carbon sources (e.g. lactate, propionate) to acetate and its fermentation by acetoclastic methanogens. It is possible that this was also related to the observation of the various metabolic models due to the varied methanogenic community composition.
Highlights
Anaerobic digestion (AD) is a microbiological decomposition of high molecular organic compounds yielding, among others, methane and carbon dioxide [1, 2]
In order to separate the biogas components, approximately 4-5 cm of the gas were injected on the vacuum line through a te on septum
The biogas fraction not frozen in the presence of liquid nitrogen, was adsorbed on molecular sieves cooled with liquid nitrogen
Summary
Anaerobic digestion (AD) is a microbiological decomposition of high molecular organic compounds yielding, among others, methane and carbon dioxide [1, 2]. AD depends on a number of physical and chemical factors, including chemical composition of the substrate, nutrient concentration, fermentation temperature, pH, volatile fatty acid composition, total nitrogen content, toxic substance content, hydration, salinity and dissolved oxygen level. These factors may stimulate or inhibit the speed and e ciency of the biological conversion of organic matter [4,5,6,7]. In natural environment methane production usually follows two main paths (apart from others) [8] It is either a product of a biological decomposition of acetic acid by heterotrophic microorganisms [9] or
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